After a recent trip to the International Rose Test Garden in Portland, Oregon and sitting in on a recent program on Rose Rosette virus, my mind is currently on roses. So, Roses it is for this week's Plant of the Week! The fragrance, the beauty, the vase life, they just can't be beat from the view point of a florist and gardener like myself.

According to Missouri Botanic Garden,Rosais a genus of about 150 species of deciduous (occasionally evergreen) shrubs and climbers noted for their beautiful, often fragrant, single, semi-double or double flowers which are borne singly or in clusters on often prickly stems clad with 5-9 leaflets often having toothed margins.

There really is nothing like the beauty of a rose, but unfortunately, the prevalance of a virus that infects roses has brought about more attention to roses recently, and not for a necessarily good reason. This disease, called Rose Rosette, can be devastating to roses in your landscape if able to spread. Learn more about how to spot it below.

Rose rosette disease was first described in the early 1940s in North America. In the seven decades since, rose rosette has become established across the Midwest and is now found across most of the United States. The causal agent of the disease, a virus, was isolated and identified in 2001. This virus was named Rose Rosette Virus, or RRV.

The symptoms of rose rosette can be striking. Classic symptoms include distortion, discoloration, and overgrowth. Flower petals, flower buds, and new leaves may be elongated and twisted. Multiple small shoots may develop in clusters, forming a growth known as a witches' broom. New growth may be an abnormal red color, with numerous small prickles (thorns) developing rather than the normal fewer, larger prickles. Shoots may elongate quickly, giving the new stems a thick, succulent appearance. Unfortunately, symptoms can vary depending on the variety, age, and condition of the rose host and environmental conditions. Not all symptoms may be present in an infected host.

Symptoms of rose rosette on a rose host infected with RRV. Note the thick, succulent stems, the proliferation of small prickles, and the red color of the new growth. Photo credit: University of Illinois Plant Clinic.

Roses appear to be the only hosts for RRV. Unfortunately, a wide variety of rose groups including hybrid tea, floribunda, grandiflora, and miniature roses appear to be susceptible. Shrub roses, including the increasingly popular Radrazz roses (better known by their registered trademark name, the Knock Out® Rose), are also susceptible. The invasive weed, multiflora rose, is particularly susceptible to RRV. The virus has been used as a biological control of multiflora rose with varying success. Preliminary testing indicates that there may be some rose species that are less susceptible to RRV, but none are currently used in the ornamental industry.

RRV is transmitted from plant to plant in the environment via the eriophyid mite Phyllocoptes fructiphilus. Eriophyid mites are tiny creatures, measuring approximately 0.01 inches in length. They are invisible to the naked eye and require a strong hand lens to be observed. These mites are easily blown by wind currents. They congregate and feed on tender new growth, such as buds and emerging leaves. As they feed, the mites acquire the virus from infected plants, then transmit the virus to healthy ones. How long the rose survives after it is infected is heavily dependent on a number of other factors, including general health of the host before infection. Most roses develop symptoms within 3 months and die within 2 years of being infected. Research indicates that the virus also reduces the cold hardiness of the rose host.

No chemicals have been shown to be effective at either preventing or curing rose rosette. Miticides are not recommended as the mites can quickly re-enter an area all season long, necessitating constant miticide applications.

Cultural management to reduce the spread of the virus includes the following: plant and propagate only virus-free plants; when planting roses, leave enough room between plants that they will not be overly crowded, even when mature; sanitize pruners between cuts; remove unwanted roses, plants that can act as hosts to the virus; and scout rose plantings for early signs of the virus. Unfortunately, many of the telltale symptoms of infection are difficult to distinguish on Radrazz roses as they typically grow quickly and their new growth usually has a red hue.

Symptoms of RRV on a Radrazz rose. Note the red discoloration of the new growth and the terminal witches' brooms. Photo credit: Diane Plewa

Once the virus has gained entry to the host, it becomes systemic and spreads throughout the entire rose plant. If a plant is suspected of being infected with RRV, the entire plant including the root ball should be removed from the landscape as soon as possible. Wrap the above-ground portion of the plant in a plastic bag before removing it to reduce the spread of the mite vector. The infected plant material (above- and below-ground tissue) can be wrapped in a black garbage bag and left in the sun for several days to kill the plant and any mites that may be present. The infected plant material should be burned or removed from the landscape. If the roots are not completely removed, they may send up new shoots. These should be immediately destroyed.

The virus has been found to persist in root fragments left in the soil; however, research indicates that the virus cannot survive in the soil alone. Plants other than roses can be planted into areas affected by rose rosette immediately while roses should not be replanted until all root fragments are removed or decomposed. (Diane Plewa)

Check out the University of Illinois Plant Clinic's Factsheet on Rose Rosette here.

If you're need more information about anything related to roses, the Our Rose Garden website has it all!

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